Loss and recovery potential of marine habitats: an experimental study of factors maintaining resilience in subtidal algal forests at the Adriatic sea.

Perkol-Finkel S, Airoldi L - PLoS ONE (2010)

Bottom Line:
Clearing and transplantation experiments subsequently demonstrated that at such advanced stages of ecosystem degradation, increased substratum stability would be essential but not sufficient to reverse the loss, and that for recovery to occur removal of the new dominant space occupiers (i.e., opportunistic species including turf algae and mussels) would be required.Lack of surrounding adult canopies did not seem to impair the potential for assisted recovery, suggesting that in these systems recovery could be actively enhanced even following severe depletions.We conclude that the loss of complex marine habitats in human-dominated landscapes could be mitigated with appropriate consideration and management of incremental habitat changes and of attributes facilitating system recovery.

Background: Predicting and abating the loss of natural habitats present a huge challenge in science, conservation and management. Algal forests are globally threatened by loss and severe recruitment failure, but our understanding of resilience in these systems and its potential disruption by anthropogenic factors lags well behind other habitats. We tested hypotheses regarding triggers for decline and recovery potential in subtidal forests of canopy-forming algae of the genus Cystoseira.

Methodology/principal findings: By using a combination of historical data, and quantitative in situ observations of natural recruitment patterns we suggest that recent declines of forests along the coasts of the north Adriatic Sea were triggered by increasing cumulative impacts of natural- and human-induced habitat instability along with several extreme storm events. Clearing and transplantation experiments subsequently demonstrated that at such advanced stages of ecosystem degradation, increased substratum stability would be essential but not sufficient to reverse the loss, and that for recovery to occur removal of the new dominant space occupiers (i.e., opportunistic species including turf algae and mussels) would be required. Lack of surrounding adult canopies did not seem to impair the potential for assisted recovery, suggesting that in these systems recovery could be actively enhanced even following severe depletions.

Conclusions/significance: We demonstrate that sudden habitat loss can be facilitated by long term changes in the biotic and abiotic conditions in the system, that erode the ability of natural ecosystems to absorb and recover from multiple stressors of natural and human origin. Moreover, we demonstrate that the mere restoration of environmental conditions preceding a loss, if possible, may be insufficient for ecosystem restoration, and is scarcely cost-effective. We conclude that the loss of complex marine habitats in human-dominated landscapes could be mitigated with appropriate consideration and management of incremental habitat changes and of attributes facilitating system recovery.

Mentions:
Clearing experiments demonstrated that Cystoseira has a high recruitment potential, but this is limited by competition with the new space occupiers. On control un-manipulated plots, which were covered by mixed mosaics of mussels and algae of low structural complexity (Fig. 6a), there was virtually no recruitment of Cystoseira (Fig. 6b). Conversely, any cleared plot was rapidly covered with recruits of Cystoseira (Fig. 6c–d), which reached densities as high as 133 juveniles per 30×30 cm (Fig. 7). The plots used in this experiment were all located on stable eroded rocky platforms which were not damaged by the severe winter storms. After an initial decrease in density, the remaining recruits in the experimental plots have survived the winter and grew to young adults. There were no differences related to the time of clearing (Fig. 7, F1,32 = 0.02, P>0.05) indicating that the potential for recruitment is high throughout the reproductive season of Cystoseira. Similarly, there were no differences in recruitment between plots cleared at shallower areas with presence of surrounding adult canopies and plots cleared in deeper areas without adult canopies (Fig. 8, no effect of Depth F1,32 = 1.63, P>0.05), indicating that the lack of canopies at deeper sites is not related to dispersal limitation.

Mentions:
Clearing experiments demonstrated that Cystoseira has a high recruitment potential, but this is limited by competition with the new space occupiers. On control un-manipulated plots, which were covered by mixed mosaics of mussels and algae of low structural complexity (Fig. 6a), there was virtually no recruitment of Cystoseira (Fig. 6b). Conversely, any cleared plot was rapidly covered with recruits of Cystoseira (Fig. 6c–d), which reached densities as high as 133 juveniles per 30×30 cm (Fig. 7). The plots used in this experiment were all located on stable eroded rocky platforms which were not damaged by the severe winter storms. After an initial decrease in density, the remaining recruits in the experimental plots have survived the winter and grew to young adults. There were no differences related to the time of clearing (Fig. 7, F1,32 = 0.02, P>0.05) indicating that the potential for recruitment is high throughout the reproductive season of Cystoseira. Similarly, there were no differences in recruitment between plots cleared at shallower areas with presence of surrounding adult canopies and plots cleared in deeper areas without adult canopies (Fig. 8, no effect of Depth F1,32 = 1.63, P>0.05), indicating that the lack of canopies at deeper sites is not related to dispersal limitation.

Bottom Line:
Clearing and transplantation experiments subsequently demonstrated that at such advanced stages of ecosystem degradation, increased substratum stability would be essential but not sufficient to reverse the loss, and that for recovery to occur removal of the new dominant space occupiers (i.e., opportunistic species including turf algae and mussels) would be required.Lack of surrounding adult canopies did not seem to impair the potential for assisted recovery, suggesting that in these systems recovery could be actively enhanced even following severe depletions.We conclude that the loss of complex marine habitats in human-dominated landscapes could be mitigated with appropriate consideration and management of incremental habitat changes and of attributes facilitating system recovery.

Background: Predicting and abating the loss of natural habitats present a huge challenge in science, conservation and management. Algal forests are globally threatened by loss and severe recruitment failure, but our understanding of resilience in these systems and its potential disruption by anthropogenic factors lags well behind other habitats. We tested hypotheses regarding triggers for decline and recovery potential in subtidal forests of canopy-forming algae of the genus Cystoseira.

Methodology/principal findings: By using a combination of historical data, and quantitative in situ observations of natural recruitment patterns we suggest that recent declines of forests along the coasts of the north Adriatic Sea were triggered by increasing cumulative impacts of natural- and human-induced habitat instability along with several extreme storm events. Clearing and transplantation experiments subsequently demonstrated that at such advanced stages of ecosystem degradation, increased substratum stability would be essential but not sufficient to reverse the loss, and that for recovery to occur removal of the new dominant space occupiers (i.e., opportunistic species including turf algae and mussels) would be required. Lack of surrounding adult canopies did not seem to impair the potential for assisted recovery, suggesting that in these systems recovery could be actively enhanced even following severe depletions.

Conclusions/significance: We demonstrate that sudden habitat loss can be facilitated by long term changes in the biotic and abiotic conditions in the system, that erode the ability of natural ecosystems to absorb and recover from multiple stressors of natural and human origin. Moreover, we demonstrate that the mere restoration of environmental conditions preceding a loss, if possible, may be insufficient for ecosystem restoration, and is scarcely cost-effective. We conclude that the loss of complex marine habitats in human-dominated landscapes could be mitigated with appropriate consideration and management of incremental habitat changes and of attributes facilitating system recovery.